1. Field of the Invention
This invention relates to electronic circuits, and more particularly to electronic communication devices.
2. Description of the Prior Art
The transmission of intelligence between distant points not directly connected by an electrical conductor may be accomplished by a radio cummunications system. A typical radio communication system has at least two fundamental components: a transmitter which converts sound variations into electrical variations; and a receiver which converts the electrical variations back into sound. Information was transmitted from a transmitter to a receiver over a specific frequency or channel. Many channels were used for the transmission of information.
One of the techniques employed by the prior art for transmitting additional information over the same channel was frequency division multiplexing. Frequency division multiplexing permits the simultaneous transmission of two or more signals over a single radio frequency. Each channel of a frequency division multiplex system had a separate crystal oscillator. The crystal oscillators were not interchangeable. Thus, if you had a seventy-two channel system, seventy-two different crystal oscillators would be needed to operate the system and at least seventy-two different crystal oscillators would have to be on hand as spares in case a crystal oscillator was defective. As the prior art advanced, frequency synthesizers were developed to replace crystal oscillators. Frequency synthesizers were interchangeable between channels. The above would also apply for frequency division multiplexed systems that utilize a cable for transmission.
Some frequency division multiplex system replaced the crystal oscillators with frequency synthesizers which were interchangeable between channels. The frequency synthesizers were phase ambiguous. Thus, frequency division multiplex systems utilizing frequency synthesizers do not provide a predictable phase shift between their inputs and outputs. An example of the foregoing problem would be when a receiver was in a vehicle travelling between two transmitters on a winding road that may be surrounded by hills and/or other obstructions, the receiver would switch back and forth between each transmitter and the person listening to the receiver would notice clicks every time the receiver switched from one transmitter to another transmitter. Prior to Applicant's invention, there was no frequency division multiplex communication system that utilized frequency synthesizers while providing a predictable phase shift between input and output. Hence, a disadvantage of the prior art was that when a frequency division multiplex system having frequency synthesizers was used in a transitional range between two transmitters, the system would not automatically steer back and forth between each transmitter, so that objectionable sounds would be heard in the audio.